Tamil Tiger rebels launched a bloody raid on a Sri Lankan naval base on the Jaffna peninsula in the north of the country yesterday, sparking a gun battle which left at least 22 dead.

Hours later in the country's capital, Colombo, a bomb exploded near an army bus, claiming the life of a soldier and wounding six people. The bomb was attached to a motorcycle parked in a crowded shopping area near the port.

The violence appears to have ended an internationally brokered peace agreement between the government and the rebels. The undeclared war has claimed more than 5,000 lives in 18 months and hundreds of people have "disappeared".

Britain halted debt relief to Sri Lanka this month in response to the government's human rights record. Germany stopped aid last December and the US has also voiced its anger over the "deterioration in Sri Lanka's human rights record".

Both sides gave conflicting reports over the battle in the north of the country, which centred on a small navy camp in the village of Delft, just after midnight. It appears that a small group of Tamil Tiger fighters had attacked the camp with backup from a flotilla of rebel ships.

A spokesman for the rebels, Rasiah Ilanthirayan, said Tiger forces had killed 35 sailors and lost four of their own fighters. "The fighting lasted only 20 minutes and we completely overran the camp," he told the Associated Press. The rebel forces then fled the scene by boat, he said. "Our attacking unit counted 35 bodies during the time they were in [the camp] but had to come back quickly."

However, an army spokesman, Brigadier Prasad Samarasinghe, said only four sailors had been killed and four wounded, and 18 Tigers killed.

He said: "The fighting at the camp lasted just one hour but the sea battle went on from 1.30am until 6am. With the air force we destroyed four of their boats completely."

The violence took place a day after the International Committee of the Red Cross announced it had pulled out staff from checkpoints between government-held and rebel areas in northern Sri Lanka. Without the presence of the ICRC, the military will shut down the checkpoints, stopping aid workers and ceasefire monitors from travelling between rebel-held and government areas.

The rebels have fought the government since 1983 to create a separate homeland for the ethnic Tamil minority, who have suffered decades of discrimination under the Sinhalese-dominated government. Nearly 70,000 people have died in the conflict, including about 5,000 killed since December 2005.